Green Bay ( green BAY) is a city in the U.S. state of Wisconsin. The county seat of Brown County, it is at the head of Green Bay, a sub-b…Green Bay ( green BAY) is a city in the U.S. state of Wisconsin. The county seat of Brown County, it is at the head of Green Bay, a sub-basin of Lake Michigan at the mouth of the Fox River. As of the 2020 Census, Green Bay had a population of 107,395, making it the third-most populous city in Wisconsin, after Milwaukee and Madison, and the third-largest city on Lake Michigan, after Chicago and Milwaukee.